Masonry cement, commonly referred to as mortar, is a cement composition prepared for use by masons in accordance with the standards of the American Society for Testing and Materials (ASTM) or its equivalent in foreign countries. The basic constituent of mortar is an hydraulic cement such as portland cement to which is added hydrated lime, fine graded aggregate such as sand and water.
Mortar is used as the bedding material in unit masonry and for coating, parging and plastering of concrete blocks and brick walls. Mortar is also used for stuccoing vertical surfaces. To satisfy the ASTM requirements, the components of mortar when mixed together must provide certain working and handling requirements of which the following are the most important: workability, plasticity, water retentivity, green strength and cohesiveness.
Present day masonry mortars depend upon the lime component in the mixture to achieve, in particular, the degree of workability and plasticity required by the ASTM standards. However, the use of hydrated lime will reduce the compressive strength of the mortar and cause efflorescence. The latter is attributable to leaching of calcium hydroxide and/or its salts and its presence will substantially reduce the aesthetics of the masonry structure. In addition, hydrated lime is alkaline in pH and can cause skin and eye irritation. Because of the caustic properties of calcium hydroxide and the environmental dangers from the leaching of lime salts, the building trades have long sought an inexpensive masonry cement composition without lime.
Similarly, concrete is formed from an admixture of cement, aggregate and water. As in mortar, the cement component must be an hydraulic cement, such as portland cement, which reacts upon contact with water to form an insoluble bond with the aggregate particles. Thus if the aggregate contains only natural or manufactured sand, the mixture is called a mortar composition and if the aggregate contains both sand and gravel, the mixture is called concrete. Manufactured sand is defined in ASTM C-144 for use in masonry mortar as the product obtained by crushing stone, gravel or air-cooled iron blast furnace slag specially processed to assure particle shape as well as gradation. The aggregate should be graded in accordance with sieve size with no particles of less than 150 microns (No. 100 sieve) present when natural sand is used as the aggregate.
The physical properties of cured mortar and concrete depend on the water cement ratio and the degree of hydration of the cement. The components of the mortar or concrete are combined with water to form an admixture in the form of a paste. During parging of the mortar or laying of block or brick slumping or sagging of the mortar can occur. This is due to dilatency and lack of thixotropy in the mixture. During casting or pouring of concrete some bleeding of water to the top surface may occur. The degree to which the mix gives off free water is a measure of the dilatency of the admixture. Dilatency is exhibited by separation of the fine particles of portland cement and fine aggregate particles permitting the lighter particles to rise to the top surface with the free water and with the heavier particles, i.e., coarse aggregate settling to the bottom. The dilatency of the admixture results in an alteration in the water cement ratio which, in turn, affects the cure properties and will result in a flawed product.